This invention relates to a process for preparing a porous amorphous silica-alumina refractory oxide, particularly for preparing porous amorphous silica-alumina oxides of controlled pore size via the sol-gel route. These products are intended for use as separation membranes, particularly for the separation of polar fluids, such as carbon dioxide or water from less polar fluids such as methane.
There are a number of mechanisms by which fluid mixtures can be separated by a porous membrane. When the pores in the membrane are larger than the largest molecular diameter of the components in the fluid mixture by a factor of up to 5 times as large, separation can occur predominantly by differences in the adsorptive interactions of the molecules to be separated with the surface of the membrane. For example, transport via surface diffusion relies on a high adsorption capacity for some gases compared to others. Pores of this size are referred to as micropores, and typically have diameters of about 3 to 20 Angstroms. Diffusion in the gas phase through pores having diameters approaching the mean free path dimensions of the molecules in the gas mixture is often termed Knudsen flow or Knudsen diffusion. Pores of this size are referred to as mesopores and typically have diameters of about 20 to 500 Angstroms (see Sing K S W et al, Pure and Applied Chem., 57 pp 603 et seq, 1985). Knudsen and laminar flows would be the predominant transport mechanisms in pores of this diameter depending on the pressures and temperatures used to operate a membrane made with pores of these dimensions.
Membranes suitable for the separation of polar gases should, in principle, separate these gases predominantly via a surface diffusion method. For surface diffusion to predominate during transport of the gases through the membrane, three criteria should be fulfilled:
(i) Pore diameters must be of molecular dimensions; PA1 (ii) The materials must be porous; and PA1 (iii) A high adsorption capacity for polar gases compared to less polar gases should be manifest.
If pore diameters are larger than the micropore range, for example as in mesoporous materials, then flow through the membrane will have contributions from Knudsen diffusion and laminar flow. The latter is non-separative and the former, at best, separates on the basis of molecular velocities (as an approximation the ideal separation factor is calculated from the square root of the reciprocal of the molecular masses of the molecules). Wholly microporous membrane materials are therefore essential if the contribution to membrane transport from surface diffusion is to be optimised. Under this circumstance, separation of a polar gas from a less polar one, for example CO.sub.2 from a mixture with CH.sub.4, should be maximised. This is the reason for developing methods of making microporous oxides that have the properties listed above.
To deposit a membrane it is essential to have a stable colloid (sol) which contains the precursor of the oxide to be deposited. On heating, this precursor is converted to the oxide which forms the membrane. For a sol to be suitable for membrane manufacture it should preferably have a viscosity between 1 and 10 mPas. Viscosity of the sol is measured by using a Contraves Rheomat 30 viscometer, at a temperature of 23.degree. C., at shear rates of 370, 684 and 1264 sec.sup.-1. The average viscosity at these three shear rates is quoted. The sol must not flocculate or gel for several months, if at all. Gelling means a viscosity increase on storage. At room temperature, once the viscosity rises, gelling may occur within 7 days.
There is therefore a need to provide a process whereby a porous amorphous silica-alumina refractory oxide of desirable properties can be produced by the calcining of a stable sol of suitable viscosity.
We have now found that this objective can be achieved by the process of the present invention.